Fuel cell systems, in particular those used for mobile applications, can be supplied with hydrogen by reforming hydrocarbons such as, for example, methanol, gasoline or diesel. In addition to hydrogen, the product gas formed in a reforming process also contains carbon monoxide, carbon dioxide and steam. In particular, the carbon monoxide has to be removed for use in the fuel cell, since this gas acts as a catalyst poison and leads to losses of power in the fuel cell.
Membranes, which may consist of various materials, such as, for example, ceramic, glass, polymer or metal, have long been used to separate off hydrogen. Metal membranes are distinguished by a high selectivity for hydrogen and a high thermal stability but have relatively low permeation rates.
To achieve a desired permeation rate, a large number of membrane cells each having a hydrogen-selective membrane are used, with the hydrogen-containing reformate gas flowing onto the individual membranes either in series or in parallel. The membrane cells are stacked on top of one another in order to form a compact membrane module.
Membrane modules onto which gas flows in series are described, for example, in U.S. Pat. No. 5,498,278 and U.S. Pat. No. 5,645,626.
A membrane module onto which gas flows in parallel, in accordance with the preambles of patent claims 1 and 14, is known from WO 01/70376. Each membrane cell of the membrane module includes a plurality of oval or approximately rectangular frames stacked on top of one another as supports for hydrogen-selective, planar membranes and for an air-permeable spacer layer for discharging permeate gas, and also two feed frames, which surround feed spaces for reformate gas. All the frames have identical external dimensions and form a compact stack with smooth external surfaces. The frames include holes which are aligned with one another and form passages for the common supply and/or discharge of the process gases, namely on the one hand to supply hydrogen-containing reformate gas from an upstream reforming process, and secondly for discharging the raffinate gas, i.e. the hydrogen-depleted reformate gas, and thirdly for discharging the permeate gas, i.e. the hydrogen which is diffused through the membranes.
Such a membrane module onto which the gas flows in parallel is a very much simpler construction than a membrane module onto which the gas flows in series, since there is no need for structures which divert the permeate gas from cell to cell, as is required when the gas is guided in series.
Nevertheless, the outlay involved in producing the membrane module which is known from WO 01/70376 is considerable, since the gases are diverted within the various frames. The holes in the frames have to be produced with a high degree of accuracy, since any projecting or recessed frame parts or burrs impede the flow of gas and may make it more difficult to produce a gastight seal. An even more serious problem is that manufacturing-related inaccuracies may lead to different magnitudes of partial flows through the individual membrane cells, which has an adverse effect on the permeation rate, as will be explained in more detail below. Finally, the frames have to be connected to one another in a gastight manner over the entire surface in order for the passages and the separation spacers to be reliably separated from one another in a leaktight manner.